1. Field of the Invention
The present invention relates to a method for separating and recovering an objective component (valuable substance or impurity such as a salt), that is widely practiced in the fermentation industry, the pharmaceutical industry, the sugar manufacturing industry, the protein and amino acid industry, the food industry, the dye industry, the pigment industry, the chemical industry, and the like.
More specifically, it relates to a method for efficiently separating and recovering an objective component from a crude liquid containing the objective component through a combination of washing and membrane treatment.
2. Discussion of the Background
The fields of the fermentation industry, the protein and amino acid industry, and the pharmaceutical industry involve production of amino acids, brewing, production of antibiotics, and the like. For example, a product liquid containing a valuable substance, such as an amino acid, is separated from a fermentation broth by means of a separation membrane, a centrifugal separator or the like, and an objective component (valuable substance) is concentrated by evaporation using steaming or the like from the product liquid; on the other hand, microbial cells still remain as a separated residue.
In the fields of the dye industry, the pigment industry, the chemical industry and the like, salting out is carried out in the production process, followed by removal of a salt as an impurity.
Recovery of a valuable substance from the separated residue is often conducted by membrane treatment instead of conventionally adopted filtration. The membrane treatment is also used for removing the salt after salting out.
Taking the fermentation industry and the protein and amino acid industry, for instance, after a product liquid (valuable substance-containing liquid) is separated from the fermentation broth, the residue still containing the valuable substance together with concentrated microbial cells is treated with a separation membrane under pressure to pass the valuable substance across the membrane to the other side, i.e., a permeated liquid side. The permeated liquid is combined with the product liquid and further worked-up by, for example, evaporation to recover the product (valuable substance).
The term "valuable substance" as used herein means a useful substance to be recovered in the above-mentioned various industries, such as an amino acid that is separated from a fermentation liquid or the like, and includes not only a final product but an intermediate product therefor. In what follows, a liquid containing a valuable substance will be sometimes referred to as a product liquid.
Since a microbial cell suspension is usually concentrated to have an increased viscosity when it is separated from a product liquid (valuable substance-containing liquid), it is difficult to treat with a separation membrane as such. Therefore, it is a generally followed practice to add water to the concentrated cell suspension thereby to reduce the viscosity to a preferred level while giving consideration to the flux value (l/m.sup.2 .multidot.hr) of a finally recovered product liquid and also to diffuse and dilute the residual valuable substance throughout the liquid to facilitate the membrane treatment. Conventional apparatus used for recovering a valuable substance remaining in a concentrated cell suspension by membrane treatment include a batch system shown in FIG. 5(a) and a continuous system shown in FIG. 6(a).
In the batch system of FIG. 5(a), a given amount of a concentrated cell suspension (crude liquid 2) containing a valuable substance is fed to container 1 equipped with a stirring mechanism, and washing water 5 is supplied thereto continuously.
A part of the liquid in container 1 is introduced into membrane separation unit 4 by means of pump 3, where a product liquid is recovered and separated through the membrane as a permeated liquid 6 (recovered product liquid) while the unrecovered liquid 7 is returned to container 1. Because the liquid level in container 1 is lowered as the recovered product liquid is separated, it is monitored with a liquid level indicator (not shown), and container 1 is replenished with washing water 5 so as to maintain the liquid level constant. Thus, most of the valuable substance in the concentrated cell suspension diffuses into the liquid and passes into the permeated liquid side through the membrane separation unit.
In the system of FIG. 5(a), while the crude liquid is fed batchwise, the washing water 5 is fed continuously. At the end of the process, the microbial cells are removed from container 1 at exit 8. Accordingly, in a strict sense, this system is not included under the category of a batch system, but a semi-batch system.
To use a plurality of membrane separation units in series to recover a valuable substance from liquid has been long known as disclosed in JP-B-59-18088 (the term "JP-B" as used herein means an "examined Japanese patent publication").
FIG. 6(a) shows an example of such a multi-stage continuous system, in which sets each stage, comprising container 1 equipped with a stirring mechanism, pump 3 for discharging part of the liquid of container 1 and returning the rest of the liquid to container 1, and membrane separation unit 4 are arranged in series, and the liquid is treated in multiple stages.
Compared with a semi-batch system when repeatedly used several times, the continuous system having the same stages has a lower recovery of a final product (valuable substance) and requires more washing water 5, resulting in a lower valuable substance concentration in the recovered product liquid 6. It follows that the recovered product liquid requires more energy for the working-up procedure, such as concentration, to obtain the final product (valuable substance). At the end of the process, the microbial cells are removed from the system at exit 8.
On the other hand, in the dye industry, the pigment industry, the chemical industry and the like, salting out is carried out in the production step. After salting out, the salt as an impurity is removed in the same manner, in theory, as for the above-described recovery of a valuable substance remaining in a concentrated cell suspension. That is, while washing water is added to a dye-containing crude liquid or the like to diffuse and dilute the salt into the liquid, the thus-obtained liquid is subjected to membrane treatment to remove the salt.
The difference from the recovery of a valuable substance remaining in a concentrated cell suspension is that the objective component to be transferred to the permeated liquid side is not a valuable substance but an impurity (e.g., a salt) which should be disposed in principle and that the residual liquid (concentrated liquid side) is a valuable substance 6 (e.g., dye) as illustrated in FIGS. 5(b) and 6(b), the system shown in FIG. 5(b) being similar to the system shown in FIG. 5(a) and the system shown in FIG. 6(b) being similar to the system shown in FIG. 6(a).
Accordingly, the-membrane treatment for impurity removal can be carried out in a semi-batch system or a continuous system similarly to that for recovery of a valuable substance. However, the conventional systems, either a semi-batch system or a continuous system, involve use of a large quantity of washing water, which results in an increased amount of drainage and necessitates large-sized installation for drainage disposal. In addition, an appreciable amount of the valuable substance (e.g., dye) migrates to the permeated liquid side, leading to a product loss.